extcon: Deduplicate code in extcon_set_state_sync()

Finding the cable index and checking for changed status is also done
in extcon_set_state(). So calling extcon_set_state_sync() will do these
checks twice. Remove them and use these checks from extcon_set_state().

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>